This invention relates to a process for forming a phosphor screen on a cathode-ray tube faceplate, and particularly to a process for forming a phosphor screen by using phosphors which are susceptible to hydrolysis or dissolution such as alkaline earth metal sulfide phosphors by a settling process.
As processes for forming phosphor screens uniform in quality on cathode-ray tubes for televisions, X-ray fluorescent tubes (e.g., image intensifier tubes), and other cathode-ray tubes, there has been employed a settling process. According to the settling process, the phosphor screen uniform in quality is formed by placing in a bulb (or a tube) a so-called cushion solution which is an aqueous solution of electrolyte such as barium acetate, barium nitrate, etc., having a pH of less than 9, pouring into the bulb a phosphor suspension comprising prescribed amounts of phosphor particles, pure water and a dilute water glass solution or a mixed solution thereof, allowing the resulting mixture to stand, while a part of water glass is gelled and adhered to the phosphor particles, which are settled out of the suspension.
An explanation will be give more concretely referring to the attached FIG. 1, which is a cross-sectional view of essential portions showing an example of the settling process. In FIG. 1, a bulb 1 for a cathode-ray tube is set so that a faceplate panel portion la is placed down, and a cushion solution 2 such as a dilute solution of barium acetate (having a pH of less than 9) is poured into the bulb 1 in a prescribed amount. Subsequently, a phosphor suspension comprising a prescribed amount of phosphor particles, pure water and a dispersing agent, and a water glass aqueous solution are poured into the bulb 1 separately or simultaneously or as a mixed solution thereof. Then, gellation and deposition take place in the solution in the bulb 1. By allowing the solution to stand for a few minutes to several tens minutes, a laminated film of phosphor is formed on the inner surface of the faceplate. Thereafter, the supernatant liquid is removed by a conventional declining method or suction method, followed by drying of the residual film.
The above-mentioned settling process has been employed from old due to relatively simple procedure, but it has a fatal defect in that it cannot be applied to all kinds of phosphors, particularly it cannot practically be applied to phosphors which are susceptible to hydrolysis or dissolution in water. For example, when phosphors which are easily dissolved in water or easily hydrolyzed, such as alkaline earth metal sulfide phosphors having as main bodies alkaline earth metal sulfides, rare earth phosphors having as main bodies alkali metal compounds (e.g., LiYO.sub.2, NaYO.sub.2, KYO.sub.2, etc.) and other phosphors having as main bodies LaOBr, BaFCl, etc., or complex phosphors thereof, are used for forming phosphor screens by the settling process mentioned above, luminescence properties (brightness) are remarkably lowered or extinguished so as to make practical application impossible. Reasons for such a lowering in brightness seem to be as follows:
(1) When phosphor particles contact with water, there are formed on the surfaces thereof hydroxide or oxide layers, the formation of which proceeds into inner portions thereof. PA0 (2) As a result of hydrolysis, there arises a pH change which causes chemical changes in a binder such as water glass component and the like contained in water. PA0 (3) As a result of hydrolysis, phosphor materials are ionized. The resulting ions react with a binder and the like to form non-luminescent substances on the surfaces of phosphor particles.